de omni eo quod est in peccato non fit satisfactio, quoniam hoc est impossibile. Unde et Deus non exigit satisfactionem de omni eo quod est in culpa, sed aliquid condonat per misericordiam, de aliquo requirit satisfactionem per iustitiam, ut simul sit his "misericordia et veritas"; nec unquam est satisfactio, nisi praecedat condonatio, sicut nunquam est meritum nisi praecedat donum gratuitum.
Unde notandum est quod in peccato duo sunt, scilicet offensa Dei et libido deordinata. Offensa Dei est infinita, et pro illa non fit satisfactio, sed Dei misericordia illa remittit suam dando gratiam. Libido autem perversa finita est; et quia Deus iustus est, non totum remittit, sed pro illa exigit poenam condignam; et quantum ad hoc possibile est Deo satisfacere.
"Satisfaction cannot be made for everything that is in sin, because this is impossible. Whence God does not demand satisfaction for everything that is [involved] in guilt, but he lets something go through mercy, and requires satisfaction for something through justice, that there may be at once both "mercy and truth"; nor is there ever satisfaction unless it preceded by something being let go, just as there is never merit without it being preceded by a gratuitous gift.
Whence it must be noted that there are two things [involved] in sin, namely the offense against God and the disordered desire. An offense against God is infinite, and there can be no satisfaction for it, but the mercy of God remits it by giving his grace. But a perverse desire is finite; and because God is just, he does not [simply] remit the whole, but exacts for that [the perverse will] an appropriate punishment, and according to this it is possible to make satisfaction to God."
As for how suffering accomplishes this, the old patristic idea of spiritual discipline or exercise makes sense to me. If we've gotten lazy and our muscles have atrophied, we have to put down the chips, get off the couch, and do some exercise. Our past laziness might be forgiven freely but that won't by itself reduce my craving for chips, my hatred of greens, or the pains of working out necessary to correct the damage I've done. If God expects our souls to be "fit" according to our capacity, he might forgive the offense to him and the neglect of our duty to ourselves and others in letting ourselves go, but he won't wave his hand and restore us to our former spiritual capacities without us doing some exercise of our own. If we confess our sins we'll be forgiven, but if we want our temptations to go away and to enjoy permissible things, we have to actually resist temptation and do the laudable things.